Applications of Focused-ion-beam/Focused-electron-beam Technology on the Fabrication of Nanodevices

Author:Hao Zuo Zuo

Supervisor:gu chang zhi

Database:Doctor

Degree Year:2018

Download:475

Pages:114

Size:6036K

Keyword:

As an important method of fabricating micro/nanostructures and devices with a collection of multi-function such as deposition/etch/irradiation,Focused Ion Beam/Focused Electron Beam(FIB/FEB)technology has been widely used in constructing nanoscale electronic devices,optical devices,optoelectronic devices,and biological fluid components,etc.This technology can flexibly and controllably fabricate three-dimensional(3D)micro/nano structures and devices with the nanometer resolution.Besides,with the development of the 3D electrical device and the third generation of wide bandgap semiconductor materials,research has been made on applying wide-bandgap-semiconductor nanostructure to 3D micro/nanodevices,which will impove integration and overcome the limitations of silicon-based semiconductors for optoelectronic device and high-frequency high-power devices/applications.In this dissertation,3D nanodevices,especially the 3D diamond ultraviolet(UV)detector and 3D gate-all-around diamond Field Effect Transistor(FET),were fabricated and realized by using deposition,etch and irradiation of the FIB/FEB technology.And we explored the mechanical property of nanowires,which can be used as free-space interconnects in 3D devices.That shows FIB/FEB technology has effective and controllable advantages on the fabrication of the 3D nanodevices.The dissertation consists of the following 3 chapters.Firstly,we systematically studied the electrically induced vibration phenomenon of the platinum(Pt)nanowires fabricated by Focused Electron Beam Induced Deposition(FEBID)technology.For the first time we determined the Young’s modulus and density of them.Futhermore,we studied how the Young’s modulus changes with the variation of the diameter of Pt nanowires.Based on its features of small mass and high Young’s modulus,Pt nanowires were used successfully to detect extra mass preliminarily as basic resonant units.Secondly,the nanodiamond cones were fabricated utilizing gray-scale patterns with FIB etching.The field emission properties and the influence of the UV light on the as-formed cones had been explored in the double probe scanning electron microscopy.It can be summarized that the as-formed nanocones with higher slicing numbers,due to its higher content of sp~2 amorphous carbon surfaces,showed a higher emission current and ultra-low turn-on field V=1.3 V/cm,in addition,twice higher emission current was achieved by UV illumination.It is proved that the FIB etching can be used as an effective method to realize the processing of sp~2 carbon/diamond hybrid system.Finally,stress-induced nanowires were fabricated by FIB irradiation methods,and used as free-space interconnects in the 3D gate-all-around diamond FET.The electrical performance of 3D gate-all-around diamond FET was preliminarily evaluated.The fabrication method is suitable for the large area processing of the device,and it lays the foundation for the fabrication of high frequency high power FET for integrated circuits in the future.

The High Pressure Synthesis and Physical Property Researches of Novel Quasi-one Dimensional Quantum Fuctional Materials

Author:Zhang Jun

Supervisor:jin chang qing wang xian cheng

Database:Doctor

Degree Year:2018

Download:109

Pages:150

Size:8994K

Keyword:

Due to the lower dimension comparing to two or three dimensional materials,the materials with one dimensional structures exhibit many novel physical properties.In this dissertation,several ternary transition metal chalcogenides with quasi-one dimensional structure have been discoveried and synthesized initially employing a high temperature and high pressure route.The crystal structures,electronic transport behaviors,magnetic properties and thermaldynamic properties have been studied systematically.The main results are shown as follows:(1)Research on thermoelectric materials has received considerable attention because of their potential applications in the field of energy conversion.The tin chalcogenides are reported as thermoelectric candidate materials frequently.The single crystal Ba9Sn3Te15 has been synthesized initially at high temperature and high pressure conditions.Single crystal X-ray diffraction results show that Ba9Sn3Te15crystallizes into a quasi one dimensional crystal structure.The compounds of Ba9Sn3(Te1-xSex)15 with x=0–1 are prepared by substituting Te using Se and all compounds are semiconductors.The band gap and Seebeck coefficient increase as the increasing content of Se.Ab initio calculations were also performed to study the density of states and band structures of Ba9Sn3Te15 and Ba9Sn3Se15,which are well consistent with band structure of one dimensional conductors.It is referred that SnTe6chains behaves semiconducting due to Umklapp scattering.It is critical to synthesize Ba9Sn3(Te1-xSex)15(x=0–1)single crystal and measure the anisotropic Seebeck coefficient for obtaining a high efficient thermoelectric material.(2)The coupling strength between adjacent spin chains in one dimensional materials plays an important role in the magnetic ordering temperature and electronic transport property.The distance between adjacent spin chains in BaVSe3 is 6.999?,which leads to the increased coupling interaction and the electronic structure of BaVSe3 is three dimensional.Ba9V3Se15 has been prepared at high temperature and high pressure conditions.The powder X-ray diffraction and refinement results reveal the crystallographic data and the distance between adjacent spin chains is 9.5745?.The magnetic susceptibility,heat capacity measurements and calculations confirm that Ba9V3Se15 can be regarded as an effective ferromagnetic chains composed of the repeating unit of V(2)(↓)V(1)(↑)V(2)(↓)cluster.(3)Establishing the relationship between the coupling strength between the adjacent spin chains in one dimensional materials and magnetic ordering temperature,electronic transport behavior is benefit for designing one dimensional materials according to different requirements.Ba9V3Te15 with one dimensional ferromagnetic chains has been synthesized initially employing a high temperature and high pressure route.The refined results of powder X-ray diffraction data indicate that the distance between adjacent vanadium chains is 10.1848?.The magnetic susceptibility and neutron diffraction confirm that the ferromagnetic ordering forms at 3.65 K and the moments arrange ferromagneticly along c axil.Comparing of magnetic and electronic behaviors of Ba VSe3,BaVS3,Ba9V3Te15 and Ba9V3Se15,it is found that the magnetic ordering temperature decreases and the electronic transport behaviros changes from metal to insulator as the coupling strength weakens.(4)In iron based superconductors,the iron chalcogenides usually display a larger local magnetic moment,different Fermi surface and abundant phase diagram comparing to the iron pnictides.The Ba9Fe3Se15 single crystal was grown at high temperature and high pressure conditions.The single crystal X-ray diffraction gives explicit crystal structure.The magnetic susceptibility,heat capacity and neutron diffraction measurements prove that the antiferromagnetic ordering forms at 15 K and the moments parallel to ab plane,presenting an incommensurate toroidal arrangement along c axil.Combined of high pressure X-ray measurement,Raman spectrum technique,X-ray emission spectrum and electronic transport measurement,it is found that the crystal structure of Ba9Fe3Se15 keeps stable with the pressure up to 60 GPa and a lattice contraction occurs at approximate 35 GPa,which is accompanied by the emergence of electronic transport behavior transition from insulator to metal and electronic state transiton from high spin to low spin.Further,a possible superconductivity occurs at 6 K.Therefore,Ba9Fe3Se15 is a possible novel iron based superconductor with quasi one dimensional structrue.

In-situ TEM Study of Lithium-related Solid State Electrochemical Processes at Nanoscale

Author:Sun Mu Hua

Supervisor:bai xue dong

Database:Doctor

Degree Year:2018

Download:241

Pages:108

Size:5799K

Keyword:

As an important branch of transmission electron microscopy(TEM),in-situ TEM has drawn tremendous attention from various fields ranging from materials science to chemistry and biology.In-situ TEM offers a unique capability to image the atomic structure of materials in real time under various external stimuli at high tempo-spatial resolutions while simultaneously measuring relevant properties.A variety of in-situ TEM holders have been developed to enable imaging and measurements under applied heat,stress,optical excitation,and magnetic or electric fields.Among these diverse kinds of in-situ TEM techniques,in-situ electrical probing TEM is the best choice to probe the solid-state nanoscale electrochemical reactions.Benefited from the advanced piezoelectricity control technique,we are able to realize precise control of the electrical probe,and relocate the target areas with the guidance of TEM imaging at the same time.In recent years,in-situ electrical probing TEM has achieved great progesses in the studies of electrochemical and solid-state inoics processes,enriching our fundamental understanding on electrochemical dynamic processes and microscopyic mechanism.The research project in this dissertation is based on in-situ electrical probing TEM.By taking advantage of the ultrahigh vacuum condition in TEM chamber,we systematically investigated several specific lithium-related electrochemical processes at nanoscale,including the conversion-type electrochemical reaction of Cu O nanowires,electrochemical solid-state amorphization in the immiscible Cu-Li system mediated by nanoscale size effect and in-situ electrochemical synthesis of novel two dimensional nanosheets of alkali metals.The main research and results are as follows:1.In-situ and real-time studies of the nanoscale electrochemistry of transition metal oxides is of both fundamental interest and practical relevance to applications involving electronics and ionics.Here the dynamic electrochemical lithiation process of Cu O nanowires(NWs)was investigated by in-situ TEM platform.We show that there exists an anomalous lithiation kinetics of Cu O NWs that is mediated by the specific sizes of NWs.That is,the lithiation rates of longer NWs turn out to be larger than those of the short ones,and more interestingly,these two ranges of NWs exhibit different lithiation kinetics relationships.Through systematic in-situ TEM studies,we elucidate the underlying mechanism that accounts for the observed anomalous electrochemical kinetics.For longer nanowires,the transport rate of electrons within the circuit is comparative with that of lithium ions,thus the reaction is Cu O Lie → Cu Li O.While for shorter nanowires,due to the much faster transport rate of electrons,lithium ions are reduced to form metallic lithium before arriving at the interface of unreacted Cu O,and then lithium atoms diffuse to the interface of unreacted Cu O to react with Cu O.2.As a typical immiscible binary system,copper(Cu)and lithium(Li)show no alloying and chemical intermixing under normal circumstances.Here we show that,when decreasing Cu nanoparticle sizes into ultrasmall range,the nanoscale size effect can play a subtle yet critical role in mediating the chemical activity of Cu and therefore its miscibility with Li,such that the electrochemical alloying and solid-state amorphization will occur in such an immiscible system.This unusual observation was accomplished by performing in-situ studies of the electrochemical lithiation processes of individual Cu O nanowires inside a TEM.Upon lithiation,Cu O nanowires are first electrochemically reduced to form discrete ultrasmall Cu nanocrystals that,unexpectedly,can in turn undergo further electrochemical lithiation to form amorphous Cu Lix nanoalloys.Real-time TEM imaging unveils that there is a critical grain size(ca.6 nm),below which the nanocrystalline Cu particles can be continuously lithiated and amorphized.The possibility that the observed solid-state amorphization of Cu-Li might be induced by electron beam irradiation effect can be explicitly ruled out;on the contrary,it was found that electron beam irradiation will lead to the dealloying of as-formed amorphous Cu Lix nanoalloys.3.Thickness-controlled synthesis of two-dimensional(2D)non-layered materials is of scientific significance because non-layer structured 2D materials may possess intriguing properties and advanced functions that cannot be achieved for their counterparts in other dimensionalities.Here we report for the first time the in-situ growth of ultrathin 2D Li and Na nanosheets by taking advantage of an in-situ electrochemically-driven experimental framework inside TEM.Real-time observations revealed with unprecedented details the dynamic growth of 2D Li and Na nanosheets with diverse shape evolution while maintaining 2D sheet-like structure consistently.DFT calculation reveals that the stronger interaction of O2 on {111} planes prevents growth along [111] direction and further promotes the final formation of 2D alkali metal nanostructures.Cathodoluminescence spectroscopy has been performed on freshly formed 2D Li nanosheet in a TEM setup to detect the plasmon eigenmodes.Peaks appearing in the visible range are shown to arrive from excitation of out-of-plane eigenmodes by the electron beam.The discovery confers a unique method to electrochemically synthesize 2D alkali metals.

Multi-field Modulated Phase Transition and Related Properties in Magneto-lattice Coupled Mateirals sy

Author:Liu Yao

Supervisor:hu feng xia

Database:Doctor

Degree Year:2018

Download:121

Pages:126

Size:5646K

Keyword:

Magneto-lattice coupled materials systems are usually multi-functional materials with ferroic-caloric effect(magnetocaloric,barocaloric,elastocaloric),negative thermal expansion et.al.As these function of the materials are closely related with its magnetostructural transition process.The study of internal and external fileds tuning of the magneto-structural transition and further the related properties are meaningful and essential for promoting the application of the materials.In this thesis,Ni2In type hexagonal MM’X,Ni2Mn1-xInx Heusler and La(Fe1-xSix)13 alloys has been fabricated through deposition of films,cold-pressing and melt-spinning technique.The influence of internal chemical atomic disorder,and residual stress introduced during the fabrication process,external field such as magnetic field,hydrostatic pressure and electric field induced strain on the phase transition,magnetism and the related properties of these materials systems has been systematically researched.The main results are shown as follows:1.Fabricating low-dimension films and realizing the magnetic refrigeration in nano-scale is one of the new developing direction of magnetic refrigeration.In this article,the deposition of Mn-Co-Ge-In films using the pulsed laser deposition technique has been explored.The study shows that the films grown retain the first order nature,however,the residual stress field and grain size effect broaden the transition of the alloy films.Compared to the magnetic caloric effect that has been reported in meta-magnetic films,the present Mn-Co-Ge-In films exhibit comparable magnetic entropy change.Besides,the films deposited on(0001)-Al2O3,(001)-SrTiO3,(001)-LaAlO3 substrates denote tunable thermal expansion behavior with particle size of the films.Specifically,Mn-Co-Ge-In/Al2O3 shows negative thermal expansion coefficient of-6.56?10-6/K(270 K390 K)while Mn-Co-Ge-In/LaAlO3 denotes ultra-low thermal expansion of-2?10-7/K(290 K390 K).The growth of films could thoroughly avoid the brittleness nature of the alloy and provide research foundations for the growth of MnCoGe based alloys films.The method gives a new train of thought on the fabrication of films with required thermal expansion coefficient,by utilizing the sensitiveness of the magneto-structural transition to the strain and defects,one could realize the quantified modulation of negative thermal expansion.2.By fabricating MnCoGe based alloys through the cold-pressing and melt-spinning method,the residual strain modulated transition behavior and magnetocaloric effect has been systematically studied.The Mn1-xCoGe ribbons has been fabricated through melt-spinning technique,the influence of grain size,grain boundaries defects and residual stress field on the phase transition and magnetocaloric effect of the alloy has been researched.Melt-spinning process could stabilize the hexagonal structure and thoroughly suppress the martensitic transition of the alloy.Annealing at 850℃for 15min could promote the diffusion of atoms to its equilibrium sate,release the residual stress,refine grains,thus induce martensitic phase in the alloy and resume magneto-structural transition.Accompanying the magneto-structural transformation,the melt spun Mn0.99CoGe and Mn0.98CoGe ribbons denote magnetic entropy change of 19 J kg-1K-1 and 18 J kg-1K-1.As a potential magnetic refrigerant,MnCoGe based alloys has bad mechanical properties and is difficult to shape up.Otherwise,the magnetic refrigerant usually require thin slices with thickness lower than 1 mm or spheres with diameters of 200400?m.Based on which,we have fabricated Mn-Co-Ge-In thin slice with thickness of 0.5 mm and diameter of 0.5 cm through cold pressing under the pressure as high as5 GPa.The residual stress in the pressed thin slice result to the broadening of the magnetostructural transition of the alloy.Characterization of magnetocaloric effect denote that the parent alloy particles,thin slices pressed under 3GPa and 5 GPa own magnetic entropy change of 30 J kg-1K-1,8 J kg-1K-1,and 4 J kg-1K-1,meanwhile,the refrigerating temperature range are 10 K,54 K and 73 K respectively.3.The transition of the alloy is not only related to the valence electron of the alloy,chemical atomic order and grain size also have great influence on the transition process and the related properties of the alloy.Up to now,however,the atomic order and grain size effect are not well understood and the reported result are not uniform.The reason could be ascribed to the fact that the grain size and atomic order always mutually changes,which inhibit the distinguishing of the effect of the two parameters.In this work,the grain size and atomic order are tuned through changing the spinning speed combining post annealing.The atomic order of the obtained ribbons are quantified through the method raised by Webster,combining the magnetic characterization,the influence of the atomic order and grain size on the martensitic transition were ascertained.The results denote that increasing spinning speed could reduce the atomic order and raise the martensitic transformation temperature.The atomic order for B2phase of 15 m/s,30 m/s and 45 m/s ribbons are 0.80,0.75 and 0.59,while the transition temperature are 220 K,238 K and 254 K respectively.The annealing process could increase atomic order,meanwhile,the grain size increased from 3μm to 8μm for the15 m/s ribbons.The comprehensive influence shows that,for Ni-Mn-In,the reduction of atomic order results to the increase of martensitic transition temperature,and plays a more important effect when the grain size is small,meanwhile,the martensitic transition temperature increases with the grain size.Further research shows that the atomic order could effectively modulate the magneto-structural transition temperature and further the magnetic refrigerating temperature range of the alloy,the three kind of ribbons fabricated has refrigerating temperature range covering 195 K to 244 K.Besides,we have for the first time researched atomic order modulation effect of exchange bias in the alloy system,the result denote that reducing the atomic order could increase the exchange bias field,and the exchange bias field are 153 Oe,174 Oe and226 Oe for the three kind of ribbons respectively.Finally,the hydrostatic pressure influence has been researched,the result denote that the hydrostatic pressure could stabilize the martensitic phase,strengthen the ainti-ferromagnetic coupling,which raises the martensitic transition temperature and strengthen the exchange bias effect.The work ascertained the internal parameters such as atomic order,grain size and external pressure influence on the transition,magnetocaloric effect and exchange bias effect.The tuning of atomic order on the spontaneous and conventional exchange bias effect has been comprehensively studied for the first time.The result denote that the atomic order could modulate the transition and further the magnetic refrigerating temperature range and exchange bias effect,which is meaningful for promoting the application of the alloy.4.La(Fe1-xSix)13 alloy system has itinerant magnetism and considerate magnetovolume effect,meaning that the transition and magnetism are sensitive to the chemical and external field such as hydrostatic pressure,this makes it an ideal ferromagnetic material for the study of magnetoelectric effect and electric-magnetic dual field tuning of magnetism and magnetocaloric effect.Choosing La(Fe0.94Co0.06)11.8Si1.2 alloy with magnetic transition residing near room temperature,and fabricated La(Fe,Co,Si)13/PMN-PT artificial multiferroic structure,we have studied the magneto-electric effect of La(Fe,Co,Si)13 alloy.The strain effect induced by the piezoelectric PMN-PT single crystal shows the ability to modulate the magnetization of the La(Fe,Co,Si)13 thin slice bonded on it through the modulation of the energy band structure.Around the room temperature of 288 K and with a magnetic field of 0.01 T applied,an electric filed of 12 kV/cm could decrease the magnetization of La(Fe,Co,Si)13 by 5.1%.Furthermore,the artificial multi-ferroic structure owns an electric field induced reversible and permanent magnetic memory effect.5.Nd2Fe14B is a typical permanent magnetic material,in which,researching the intrinsic magnetic properties of the alloy could help to guide the optimization of the permanent magnetic properties of the alloy.Through the introduction of Ta buffer layer,the permanent magnetic Nd2Fe14B films with textured structure has been successfully grown on the(011)-PMN-PT single crystal.The influence of buffer layer,film thickness,substrates,microstructure influence on the permanent magnetic properties and spin reorientation has been studied.Furthermore,the modulation of electric field induced strain on the spin reorientation and magnetism of the deposited Nd2Fe14B films has been studied,and a new mechanism for the magneto-electric effect has been raised.Below the spin reorientation temperature of Nd2Fe14B,the dynamic strain field could influence the structure distortion of Nd2Fe14B and tuning the crystal filed coefficient,further the magnetic anisotropy of the alloy,as the spin reorientation and magnetism is intimately related to the magnetic anisotropy,the magnetization of the films can thus been modulated.When the temperature is lowered down to 30 K,with magnetic field of 2 T applied,an electric field of 6 kV/cm could decrease the magnetization of Nd2Fe14B films by 30%.

In-situ TEM Investigation of Alkali Metal Intercalation of Transition Metal Dichalcogenides

Author:Huang Qian Ming

Supervisor:bai xue dong

Database:Doctor

Degree Year:2018

Download:1116

Pages:109

Size:5403K

Keyword:

Belonging to the class of various two-dimensional materials,transition metal dichalcogenides(TMDCs)show a wide range of electronic,optical,mechanical,chemical and thermal properties and have great potential in areas ranging from nanoelectronics,optoelectronics,catalysis to energy storage.The van der Waals gap between the layers of TMDCs allows insertion of guest ions such as alkali metal.Intercalation of TMDCs is one important field in the chemistry of inclusion compounds.With guest materials intercalated in the host ones,the structure and electronic properties of the host materials can be drastically manipulated.Here we investigated the alkali metal intercalation process in TMDCs(MoS2 and WS2)via an in-situ transmission electron microscopy(in-situ TEM).TEM provides ability to correlate structural data with chemical information;as such,in-situ TEM is unique in its ability to obtain the above corelation under various additional environment in real time.By in-situ TEM we investigated the lithiation and sodiation process in MoS2 and WS2 nanosheets.The main results are as follows.1.We constructed electrochemical half-cell inside TEM,which involved MoS2naonsheet as one electrode,Li metal as the counter electrode and Li2O on the lithium as the solid-state electrolyte.The dynamic lithiation process was recoreded by in-situ TEM and the structural transformation of Mo S2 are revealed by the electron diffraction patterns.The pristine MoS2 transformed into an intermediate phase characterized by its 2×2 superstructure,during which time phase transition from 2H-1T occurred.With excessive lithium intercalated,Li2S and Mo were generated.2.We investigated the sodium intercalation process in MoS2 nanosheets via in-situ TEM and obtained real-time images and diffraction patterns of the sodiated MoS2,which helps to better understand the electrochemical process and phase transition mechanism at the atomic level.Based on the high resolution transmission electron microscopy and electron diffraction pattern,we demonstrate that MoS2 undergoes an intermediate phase that should be denoted as 2H-Na0.25MoS2,showing particular electron diffraction patterns which can be defined as 2×√3 superstructure.For more sodium insertion,2×2 patterns emerged,indexed as 2H-Na0.5MoS2 and 1T-Na0.5MoS2respectively according to the intensity distribution of the electron diffraction spots.Phase transition from 2H-Na0.5MoS2 to 1T-Na0.5MoS2 occurs,resulting from the decreased stability of 2H-MoS2 lattice and the enhanced intervalence charge transfer between Mo3+and Mo4+ion.3.By using in-situ TEM method,we investigated the lithium ions insertion process in WS2 nanosheet and obtained real-time images and diffraction patterns of the lithiated WS2.The electron diffraction patterns demonstrated that pristine WS2 transformed into2H-Li0.5WS2 with 2×2 diffraction patterns,followed by a phase transiton from 2H-1T and transformed into 1T-Li0.5WS2.With more lithium intercalated,Li2S and W were generated.Upon sodium insertion,WS2 underwent an intermediate phase 2H-Na0.25WS2 with its 2×(?) superstructure,and Na0.5WS2 with its 2×2 superstructure,which is similar to the sodiation process of MoS2.

Prediction of Several New Materials

Author:Liu Ning

Supervisor:guo li wei chen xiao long

Database:Doctor

Degree Year:2018

Download:194

Pages:110

Size:7278K

Keyword:

We design several new structure materials and predict their properties based on first principles calculations.These include two-dimensional gold in honeycomb structure and a series new materials in diamond-like structure which is constructed by X atom centered XN4 tetrahedra and N-N dimer fundamental units.The main results are as below.Firstly,we prove that two-dimensional honeycomb gold is relatively stable owing to its strong relativistic effect and electronic configuration by first principles calculations.It has covalent bond and semiconductor band structure with a gap of 0.1eV.Tailoring it into nanoribbons with the armchair type of edges,its band gap can be further widen to about 0.3 eV.In contrast,single layer of close packed plane gold is metallic.Both of them are more transparent to visible light compared with graphene.They are potential candidates in transparent conductive materials and electronic logic devices.In experiments,we try to get two-dimensional honeycomb gold by dealloying BaAu2.The deposit is nanoporous gold.Some honeycomb atomic patterns are observed in suspension.It needs further demonstration.Secondly,we construct a diamond-like structure by using Si centered SiN4tetrahedra and N=N dimers as the fundamental building units.The high density of delocalized electrons provided byπ*antibond in N=N is crucial for its high electronic conductivity.It’s stable in lattice dynamics and relatively energy favored.A striking feature is that it has free-electron energy dispersions in its band structure.It will serve as a good low-density(0.996 g/cm3)metal candidate with a high electrical conductivity(5.07×105 S/cm),a high thermal conductivity(371 W/m?K)comparable to copper,and ordered cavities(7.4?).AlN4 in isostructure is also stable and has similar band structure.The results presented here show that nitrogen dimers can be used as proper fundamental building units to design new compounds with high conductivity.Lastly,we replaces Si in SiN4 by M,where M is 3d transition metal elements.By comparing their ferromagnetic and antiferromagnetic energy,and calculating corresponding band structure,we find that FeN4,CoN4 and MnN4 are half metals with band gaps larger than 2 eV in one spin direction.They are stable in thermal dynamics and lattice dynamics.N-N bond acts as a bridge for conducting carriers and helps neighboring 3d transition metal atoms realize ferromagnetic coupling.The hybridization between N’s and M’s atomic orbitals lead to the formation of bonding and antibonding states in one spin direction and opening a band gap.Their curie temperatures are much higher than room temperature,which enables them to become candidates for practical spintronics application.

Surface Enhancement Raman and Surface Catalysis Reaction in Noble Metal Nanoparticles

Author:Yang Xian Zhong

Supervisor:liang wen jie

Database:Doctor

Degree Year:2017

Download:190

Pages:98

Size:7751K

Keyword:

Plasmons are elementary excitations of electrons in the solid relative to the background of the ion core.Local surface plasmon resonance can produce strong local electromagnetic field enhancement,so it is widely used in many aspects such as surface enhanced Raman,surface enhanced fluorescence,plasmonic catalytic reaction and so on.In this paper,the self-assembly of Au@Ag core-shell nanocuboids and its surface enhancement effect are discussed systematically.Then,the surface catalytic reaction between silver nanoparticles and molybdenum disulfide hybrid system is introduced.Finally,we discussed the fabrication and characterization of graphene ultrashort channel FET devices.The main research results are as follows:1.The self-assembly of gold nanorods has been extensively studied over the past ten years.At present,single-layer and three-dimensional nanorods assembies have been achieved,but the layer by layer arrangement is not yet achieved.We selected Au@Ag core-shell nanocuboids,and two different arrays were obtained on the same substrate by two-step droplet evaporation method,namely,the horizontal arrangement(the major axis was parallel to the substrate)and the vertical arrangement(the major axis perpendicular to the substrate),and the two arrangements are separated from each other.Horizontal arrangement is equidistant stepped structure,and the width of each step is equal,which is about 768 nm.While the widths of vertical arrangement are inequispaced,they are of several micrometers.And the height of each layer is about 75 nm.The spacing of two nanocuboids can be adjusted by removing the surfactant.The FDTD calculation results show that the reduction of the spacing can greatly improve the electromagnetic field enhancement.We used 4-MBA as a Raman probe molecule to study the enhancement of both arrays.The enhancement factor of monolayer vertical arrays can reach 3.87 × 105,and the Raman intensities increase with the number of array layers,which means the plasmons propagate well in our structure.Our layered structures have great potential applications in the fields of surface-enhanced Raman,plasmonics,solar cells,nanodevices and so on.We have also studied the arrangement of silver nanocubes in a similar way.We found that the types of surfactants had a significant effect on the arrangement of nanocubes.In addition,we also studied the phase separation phenomenon in the nanorod arrangement.2.We transfered MoS2 onto the surface of silver nanoparticles(Ag NPs)to form a MoS2+ Ag NPs hybrid system.By adjusting the size of Ag NPs,we successfully adjusted the absorption peak of the hybrid system from 435 nm to 532 nm,which resulted in resonance absorption of the hybrid system with the incident laser.We measured the enhancement effect of different sizes of Ag NPs on MoS2 fluorescence.The results showed that Ag NPs with average diameter of 6.1,14.5 and 25 nm could enhance the fluorescence of MoS2 by 2.5,13 and 52 times,respectively,which indicated that the plasmon-exciton coupling strength between MoS2 and Ag NPs increases with increasing particle size.The surface catalytic properties of MoS2 + Ag NPs hybrid system were studied by using 4NBT as the probe molecules.The results showed that the catalytic ability of the hybrid system increased with the increase of Ag NPs’ sizes.The catalytic effect comparations between the hybrid system and Ag NPs show that although the MoS2 has an attenuation effect on the electromagnetic field,the catalytic effect of the hybrid system is better.The hybrid system of MoS2 + Ag NPs can be extended to other two-dimensional materials and more metal nanostructures,which has great application in surface-enhanced Raman,fluorescence and surface catalysis.At the end of the paper,we introduced the construction of graphene ultrashort channel devices and measured the transport properties at room and low temperature.This device will have a great application in quantum transport,surface plasmonic photonics and photodetectors.

A New Improvement and Application of Gutzwiller Method

Author:Zhang Jian

Supervisor:dai xi weng hong ming

Database:Doctor

Degree Year:2018

Download:82

Pages:90

Size:1363K

Keyword:

The density function theory(DFT)successfully calculates ground state properties and band structures of many materials.However,single particle approximation fails to describe electron behavior in strongly correlated materials.Several schemes for these materials have been proposed,including LDA+U,LDA+DMFT,and LDA+Gutzwiller.LDA+Gutzwiller has been applied to strongly correlated system model studies and many strongly correlated materials.It also requires much less calculation than the LDA+DMFT scheme.However,convergence near the transition point is difficult.For materials with either large Coulomb interaction or bands near the Fermi level,convergence is particularly difficult.In this paper,we present a novel application of the Gutzwiller method by combining it with Newtons method and golden section search.Our hybrid method dramatically improves convergence.In order to help people understand this method,read and understand the code,we give detailed description of all the equations used in Gutzwiller code,and equations in Gutzwiller+Newton method,and we give a brief description of the structure of our code.In order to calculate correlated materials,we propose implementation of the LDA+Gutzwiller method combined with Newtons method.Applying Newtons method with golden section search and other improvements for the self-consistence process,efficiency and convergence of calculation are improved.We compare behavior of the previous linear mix method and this Newtons method.This combined application dramatically improved self-consistent loop efficiency,and converges cases previously cannot converged.We have applied our code to study the electronic structure of several typical strong correlated materials,including SrV O3,LaCoO3 and La2O3Fe2Se2.Our results fit quite well with previous studies.

The Research on Magnetic and Transport Properties for Heusler Alloy Mn2PtSn and Hexagonal MnNiGa Alloy Thin Films

Author:Li Yue Qing

Supervisor:liu zhong yuan wang wen hong

Database:Doctor

Degree Year:2017

Download:83

Pages:97

Size:4145K

Keyword:

Spintronics is an emerging discipline,and its vigorous development greatly promotes the progress of modern information technology.In traditional semiconductor devices,people only use its charge properties.Not only the charge properties but also its spin is contained in spintronics,which greatly enriches the research content of traditional electronics,and provides a train of thought for the development of a large number of new devices.The research of functional magnetic films is an important branch of spintronics.Recently,the discovery of the skyrmion topological magnetic structure in magnetic materials sets off a boom in magnetism research.The advantages of skyrmion,such as small size,high stability and mobility,make it hold great promise to build future high density,high speed and low energy consumption magnetic information storage devices.Due to its special spin magentic structure,skyrmion will lead to rich and innovative transport physical phenomena in the real space,such as the topological Hall effect.Researching and understanding the magnetic and spin-related transport properties will provide important theoretical and guiding significance for the realization of the next generation of new spin topology memory devices.In this paper,the Heusler alloy Mn2PtSn and hexagonal MnNiGa alloy were selected as the main research objects.The magnetron sputtering technique was used to successfully fabricate the above two polycrystalline films.The crystal structure,magnetic properties and spin related transport properties of the two thin films were studied in detail,and the magnetic origin was analyzed by the first principles calculations.The magnetic and transport properties of the Heusler alloy Mn2PtSn film were investigated.The magnetron multi-target co-sputtering technique was used to prepare the Mn2PtSn thin film.The magnetic and transport properties of the Mn2PtSn film were measured.The anomalies in magnetic properties,the resistance curve,the magnetoresistance curve and the abnormal Hall resistance were observed in Mn2PtSn film,which was different from the traditional behavior of ferromagnetic metals.A quite large topological Hall effect(THE)was observed in a wide temperature range of 10K-300K for Mn2PtSn thin film,and the value of THE reached a maximum of 0.57μΩcm around 150K,which was the largest value among the THE-hosting metallic materials so far.It was concluded that the large topological Hall resistance of Mn2PtSn was caused by the skyrmion topological spin structure.The magnetic and transport properties of hexagonal MnNiGa films were investigated.The hexagonal MnNiGa films with different deposition temperatures and different deposition thicknesses were prepared by magnetron sputtering technique,and the structure,magnetic properties and transport properties were measured.XRD results showed that MnNiGa film samples were impure and contained non-magnetic Ga3Ni2 phase.Magnetic and transport results showed that all MnNiGa film samples exhibited anomalies at T=200K,and these anomalies were largely unaffected by deposition temperature or deposition thickness.Due to the presence of the skyrmion topological spin structure in hexagonal Mn NiGa,the material occurred spin-reorientation at 200K,resulting in anomalous phenomena in magnetic and transport properties.The hexagonal pure Mn NiGa films with different Mn contents were prepared by magnetic double target co-sputtering technique,and the structures,magnetic properties and transport properties were measured.A large topological Hall effect was observed in hexagonal MnNiGa films with different Mn contents,and it existed in a wide range of temperature from 10K to 300K.The maximum value reached 143nΩcm at T=250K.Furthermore,we found that the size and the sign of the topological Hall resistance can be manipulated by adjusting the content of the Mn element,and it also showed again that there existed skyrmion in the hexagonal MnNiGa.

Research on the Preparation and Properties of Molybdenum Disulfide Films and Their Optoelectronic Devices

Author:Dan Jun Jie

Supervisor:li jin hua

Database:Doctor

Degree Year:2018

Download:348

Pages:112

Size:8825K

Keyword:

Two-dimensional layered nanomaterials represented by graphene and molybdenum disulfide(MoS2)have shown wide applications in many fields,such as electronic,photoelectric devices and sensors,due to their good stability,abundant structure and excellent physical and chemical properties.And it has gradually become the focus of research in many fields,such as physics,chemistry,material science and so on.The MoS2 nanomaterials have adjustable band gap which are different from zero band gap of graphene.The special band gap of MoS2 makes it have a wide application prospect in all kinds of functional devices.High quality and few layered MoS2 films and MoS2/WS2 heterostructure composites were prepared by mechanical exfoliation and chemical vapor deposition(CVD)in our research.The morphology,composition and optical properties of the materials were characterized and analyzed.A back gate field effect transistor(FET)based on MoS2 film of different layers and its heterostructure was successfully prepared.And the devices are applied to photodetection with high sensitivity and micro glucose biosensors with ultra low concentration.The main contents are as follows:(1)The researches on the preparation and optical properties of few layered MoS2films was carried out.High quality and few layered MoS2 films were prepared by CVD and mechanical exfoliation.The crystal structure,morphology,thickness and optical properties of the materials were also characterized.According to the contrastive analysis of the morphology of the samples prepared by the CVD method at different reaction time during the growth process,the growth mechanism of MoS2films was discussed.During the formation of S-Mo-S bonds,the growth speed of different crystal planes was different,resulting in the growth of hexagonal MoS2crystals in three directions.And finally the triangulated thin MoS2 film structure appeared.It was also found that the other TMDs films have the same morphological evolution during the growth process.The researches content of this part establish a foundation for the growth of MoS2 heterostructure and the application of MoS2 thin films in functional devices.(2)The research on the preparation and electrical properties of few layered MoS2films FET was carried out.A back gate field effect transistor based on MoS2 films of different layers was prepared.And the electrical properties of the devices were compared and analyzed.The output characteristic curve of the devices reflected the good control effect of gate voltage on the device.Moreover,a good ohmic contact was formed between the source and the drain electrode and the MoS2 channel layer,which was beneficial to the injection of the carriers.The transfer characteristics of the device showed that the carrier mobility of bilayer to five layer MoS2 films based FETs were 31.7,12.8,4.1 and 3.2 cm2/V·s,respectively,under Vds of 0.5 V.(3)The photoelectric characteristics of MoS2 based FET was studied.In consideration of the incident light irradiating the top of the device from top to bottom,a back gate phototransistor based on few-layered MoS2 films was designed and prepared.The effect of optical power and gate voltage on the photoelectric performance of the device was discussed by contrast experiments.The experimental results showed that,under the fixed gate voltage,photocurrent and photoresponse of the devices increases gradually with the increase of optical power.At the same optical power,the greater the gate voltage was,the greater the photoresponse was achieved.(4)The preparation and photoelectrical properties of MoS2 heterostructure based FET were studied.First,large area and high quality MoS2/WS2 heterostructures were prepared by CVD method,and the morphology,composition and optical properties of the heterostructures were characterized and analyzed.In the study,we discussed the morphological differences in different regions of heterostructure samples and the evolution process between different regions.By comparing the optical properties of the heterostructure with the MoS2 film,the motion of the carrier at the heterostructure interface was revealed.Then the photoelectric properties of FET based on the MoS2heterostructure were studied and compared with MoS2 devices.Under 20 mW illumination,the photoresponse of MoS2/WS2 heterostructure based phototransistors increased by 50 times.The photoresponse of MoS2/MoO3 heterostructure devices formed by the composite high work function MoO3 material increased by nearly 100times.The results showed that the MoS2 heterostructure can enhance the performance of device effectively.Nanomaterials with high work function induced the charge transfer process at the heterostructure surface to produce the built-in electric field.Furthermore,it can effectively promote the separation of photogenerated electron hole pairs in MoS2.A good photoelectric modulation effect on MoS2 film phototransistors was achieved.From the theoretical analysis of energy band,we know that the main reason for the improvement of photoelectric performance is the difference of the band location of two kinds of materials in heterostructure,resulting in the effective separation of photoelectron hole pairs at the interface.The hole was captured by the defects at the heterostructure interface,while the electronic was moved to the external circuit to participate in the electrical conductivity,which effectively improved the photoelectric performance of the device.(5)The performance of glucose biosensor based on MoS2 FET was studied.The results showed that,under the same gate voltage and drain voltage,the Ids increased with the increase of glucose concentration,so that the detection of unknown glucose solution was realized.In our research,we used the MoS2 FET for the first time to realize the ultra sensitive detection of an extremely low concentration of glucose solution.The sensitivity of the glucose biosensor was as high as 269.71 mA/mM.The detection limit was as low as 300 nM and the linear range was 300 nM30 mM.In addition,the device we prepared also had the advantages of short response time(<1s),good stability,and micro detection.This part of the study shown that few-layered MoS2 based FET had a wide development space in the field of the application of new functional devices.